Lens screens of the type referred to above are especially used where a surface must be illuminated as homogeneously as possible.
U.S. Pat. No. 3,580,675 discloses a lens screen wherein individual lens elements are arranged along a line. No spacing is provided between the discrete lens elements which are held laterally. The two optically effective surfaces are of the same size and the lens elements are not tapered. This type of attachment does not permit a surface arrangement of the lens elements since a large space laterally of the elements is needed for securing the same.
U.S. Pat. No. 4,275,962 discloses a lens screen which is used as a lens objective. In this lens screen, maximally two rows of lens elements can be held by a carrier. A mechanical support by the carrier is not provided; instead, the individual lenses can be arranged one next to the other without an intermediate space in a maximum of two rows. In addition to the rather unstable configuration at high exploitation of the incident energy, the nature of the mounting does not permit a surface arrangement of the lens elements since the carrier projects outwardly on both sides far beyond the optical surfaces which are used.
A further lens screen is disclosed in German Patent 968,430 wherein the lens elements are mounted on a planar plate. Such lens screens are today produced by injection molding or by mounting discrete lens elements on a planar glass plate with the aid of cement. In this way, it is possible to obtain a surface arrangement of lens elements with this lens screen with the lens elements being arranged very tightly next to one another. The imprecision of the optical imaging is disadvantageous in lens screens produced with the injection molding process (wherein plastic but no glass is used as a lens material). This makes it impossible to use lens screens produced in this manner in optical precision machines. If the discrete lens elements are mounted on a glass plate with the aid of cement, no limitation as to optical quality with respect to imaging must be accepted. However, it is here disadvantageous that an absorption of radiation occurs at the glass plate and further optical junctions occur at the cement layer which is disposed in the optical beam path. The enormous complexity of cementing the many discrete lens elements is a serious limitation with respect to manufacture. Because of the cement, lens screens having lens elements cemented to a transparent base plate are not usable in all areas of application since the durability of the cement with respect to chemical influences and certain irradiation ranges is very limited with respect to time.
U.S. Pat. No. 4,550,979 discloses a vacuum-tight radiation window wherein several mutually adjacent individual lenses are held in a metal holder. A high mechanical strength is needed for use in the wall of a vacuum chamber. Furthermore, this lens screen must be cooled. This configuration of the lens elements does not permit an optimal utilization of a large area incident beam.